The prescribing of a medication is the most common intervention employed by physicians in the care of geriatric patients in the office setting. However, older patients are at increased risk for drug-related iatrogenic injury due to age-related pharmacologic changes, the physiologic declines associated with aging, and the increased burden of chronic illness that characterizes the elderly patient population. In this prospective study, the occurrence of adverse drug events (ADEs) will be measured over a one- year period of observations in a large population of persons 65 years of age or older,, who are enrolled in the Fallon Community Health Plan, the first health maintenance organization in the United States to enroll Medicare beneficiaries and to accept reimbursement from HCFA on a prospective per capita basis. This study the emphasize the identification of preventable ADEs. Information on ADEs will be ascertained through computerized surveillance of pharmacy records and laboratory data,. reporting of drug-related events by healthcare providers, review of ambulatory and hospital medical records, and systematic interviews with physicians, nurse practitioners, nurses and pharmacists. This study will identify patient and healthcare provider factors associated with the occurrence of ADEs, as well as the underlying causes and systems failures that lead to their occurrences. Resource utilizations and costs associated with ADEs will also be assessed utilizing a nested case-control study design. The research will employ a multi-method approach which will include descriptive statistics and open-ended qualitative key informant interviews, utilizing structured interview instruments, to evaluate and prioritize systems failures that lead to preventable ADEs. The research design allows for the examination of factors relating to the occurrence of ADEs at the individual group, and organization level to gain insight into the causes of preventable events among ambulatory elderly patients. Information derived from this study will provide essential information to inform the design of interventions to reduce the risk for the occurrence of preventable ADEs in this vulnerable population.